The knee joint of the dog and cat, known as the stifle joint, is similar to a human’s knee. The joint is made up of the femur (thigh bone), tibia (shin bone), and the patella (kneecap) that are firmly held together by ligaments. Ligaments are strong, dense structures consisting of connective tissues that connect the ends of two bones across a joint. The function of ligaments is to stabilize the joint.
The stifle has two very important ligaments called the cranial (CrCL) and caudal (CaCL) cruciate ligaments (cruciate means a cross or crucifix) that cross in the center of the joint. The CrCL (known as the ACL in humans) restrains the backward and forward motion of the joint, in addition to inward twisting and hyperextension of the joint. It is the structure that is most commonly injured in dogs. In fact, more than 600,000 dogs in the U.S. have surgery for this problem every year.
The stifle also has two half-moon shaped cartilage structures between the weight-bearing bone ends called menisci. There are two menisci in each stifle, one on the inner side of the joint called the medial meniscus and one on the outer side of the joint called the lateral meniscus. The menisci add support to the stifle and also serve as shock absorbers by spreading the weight load.
How the meniscus tears
With heavy play or with a strong twist of the limb, the femur slides backward on the tibia bone, and the meniscal cartilage, a cushion between the bones that acts as a shock absorber, may be crushed. This type of injury is often accompanied by a “click” that can be heard when an animal walks. When the CrCL is weakened or torn, the most significant long-term change in the joint is the development of arthritis.
Clinical signs and diagnosis
Many pets with a torn meniscus usually have had a surgery to stabilize the knee weeks to years previously. Acute lameness is the typically clinical sign of a torn meniscus. Although the patient may temporarily respond to administration of pain medication, the clinical signs of lameness persist.
The meniscus is evaluated either with the use of a one-inch incision along the side of the knee or preferably with the use of an arthroscope and a few tiny incisions. The offending torn portion of the meniscus is trimmed so that it no longer pops back and forth within the knee joint.
By 4 to 6 weeks after surgery, most pets are fully weight-bearing on the operated limb; although, exercise should be limited for the first month after the procedure.
For more information on this subject, speak to the veterinarian who is treating your pet.